Most chemical warfare agents are liquids that evaporate into vapors at varying rates. As effective weapons, they would need to be widely spread by a spray or explosion indoors. Outdoors, even small breezes disperse dangerous vapors.

CDC.Textbook of Military Medicine: Medical Aspects of Chemical and Biological Warfare.
U.S. Army Surgeon General Medical NBC Information Server.
Organization for the Prohibition of Chemical Weapons.
U.S. Army Center for Health Promotion & Preventive Medicine: "Detailed Chemical Facts Sheets."